Endometriosis, a common condition affecting young women of child-bearing ages, causes considerable pain and discomfort and is associated with infertility. The economic costs of diagnosis and treatment are substantial. Even though it is a notable cause of morbidity in women, no epidemiologic investigations have been published. The purpose of this case-control study is to determine the strength of association between potential risk factors and endometriosis ascertained by laparoscopy or laparotomy. More specifically, the relationship between voluntary pregnancy deferral, years of menstrual cycling, family history, hormonal exposures, pelvic surgery and endometriosis will be examined. Furthermore, cases will be asked about certain life-style factors which may place them at increased risk of developing the disease. Their responses will be compared with those obtained from a disease-free community-based control group. Four hundred cases of external endometriosis diagnosed at The Johns Hopkins Hospital, Sinai Hospital and George Washington University Medical Center between January, 1983, and December, 1984, will be identified and interviewed after obtaining physician consent. Two types of cases will be ascertained from discharge diagnoses, operating room logs, and files specific to each hospital: one group presenting with infertility and a second group being investigated for pelvic pain. Exclusion criteria include residence outside the Baltimore or Washington areas, age less than 18 or greater than 44, no listed telephone number, and a diagnosis of adenomyosis. A random digit dial control group has been selected to minimize biases. Controls will be stratified on telephone exchange and group matched for age with the case series. Both cases and controls will be interviewed by telephone. Preliminary analyses will compare respondents with nonrespondents on the basis of medical record data. Descriptive statistics will be used to describe distributions of variables among cases and controls. Potentially confounding variables and effect modifiers will be examined. Odds ratios will be used to assess the strength of association between the outcome variable and exposure variables. We will use multiple logistic regression in a step-up fashion to describe the model that best explains the data and to yield unconfounded estimates of the strength of association between the independent risk factors and endometriosis.